Electrical cables which are used in oil wells must be able to survive and perform satisfactorily under extremely adverse conditions of heat and mechanical stress. Ambient temperatures in wells are often high and the I.sup.2 R losses in the cable itself add to the ambient heat. The service life of a cable is known to be inversely related to the temperature at which it operates. Thus, it is important to be able to remove heat from the cable while it is in its operating environment.
Cables are subjected to mechanical stresses in several ways. It is common practice to attach cables to oil pump pipes to be lowered into a well using bands which can, and do, crush the cables, seriously degrading the effectiveness of the cable insulation and strength. The cables are also subjected to axial tension and lateral impact during use.
It is therefore conventional to provide such cables with external metal armor and to enclose the individual conductors within layers of materials chosen to enhance strength characteristics of the cable, but such measures are sometimes not adequate to provide the necessary protection.
An additional problem arises as a result of down-hole pressures, which can be in the hundreds or thousands of pounds per square inch, to which the cables are subjected. Typically, the insulation surrounding the conductors in a cable contains micropores into which gas is forced at these high pressures over a period of time. Then, when the cable is rather quickly extracted from the wall, there is not sufficient time for the intrapore pressure to bleed off. As a result of this decompression, the insulation tends to expand outwardly like a balloon and can rupture, rendering the cable useless thereafter.
In U.S. Pat. No. 4,409,431 in which the assignee is the same as the assignee of the instant invention, there is described a cable structure which is particularly suitable for use in such extremely adverse environments. The structure protects the cable against inwardly-directed compressive forces and provides for the dissipation of heat from the cable which is an important feature in high temperature operating environments, for reasons discussed therein, as well as resistance to decompression expansion of the insulation. Supplemental force-resisting members for such structures are disclosed in my copending U.S. patent application Ser. No. 429,530 filed Sept. 30, 1982 and assigned to the same assignee as the present invention.
As described in U.S. Pat. No. 4,409,431 the cable protective structure includes one or more elongated force-resisting members which conform to, and extend parallel and adjacent an insulated conductor comprising the cable. The members are rigid in cross-section to resist compressive forces which would otherwise be borne by the cable conductors. For applications requiring the cable to undergo long-radius bends in service, the elongated support may be formed with a row of spaced-apart slots which extend perpendicularly from the one edge of the member into its body to reduce the cross-sectional rigidity of the member in the slotted areas so as to provide flexibility in the support to large-radius bending about its longitudinal axis.
As described in my copending patent application Ser. No. 390,308 filed June 21, 1982 and assigned to the same assignee as the present invention, for certain service applications, it may be preferred that the electrical insulating sheath on the cable conductor not be in direct contact with the slot openings. This is because the slot openings in the support member may allow highly corrosive materials to gain access to the jacket composition by flowing inwardly through the slots. In addition, the corners formed by the slots may cut into or abrade the underlying cable jacket upon repeated bending of the cable.
The cable protective structure of said copending application Ser. No. 390,308 is made of a composite structure which utilizes an elongated force-resisting member of good thermal conductivity positioned adjacent the insulating conductor sheath. This member comprises a channel member having two substantially parallel elements or legs which are cantilevered from a transverse or vertical leg and which are slotted laterally to impart the requisite long-radius bending in the plane of the vertical leg. The parallel legs may extend in the same direction from the vertical leg toward an adjacent conductor in which case the channel has a U-cross sectional shape. A smooth, bendable liner may be mounted between the three legs of the channel and the outermost layer of insulation of the adjacent conductor to bridge the slots in the member and thereby protect the underlying insulation from abrasion by the slot edges during bending of the channel member.
The exterior jacket or armor, the liners and the channel members all serve to protect the conductor insulation, and hence the cable, from damage caused by vertical crushing, horizontal or lateral (edge) impacts and from damage resulting from decompression expansion.
The vertical legs of the channel members greatly enhance crush resistance and this is true even if the width of each of the vertical channel legs of the outermost channel member is made only about one-half that of the centrally located channel member in a flat three (or more) conductor cable construction. Since the outer two channel members can be reduced in overall thickness, it permits proportionally more insulation to be enclosed by the relatively thinner channel member without necessarily increasing the overall thickness of the cable. The extra thickness of insulation can serve to provide greater resistance to edge or lateral impacts and consequently, if the cable edge is dented, it is more likely that the minimum effective thickness and integrity of conductor insulation will remain uncompromised. Also, during certain steps in the manufacturing process and particularly when the cable is being jacketed or armored with steel tape, the outside insulation may be displaced longitudinally or radially, or otherwise may be deformed. Accordingly, it is advantageous to provide an extra thickness of insulation material on the outside conductors so that the required minimum thickness of insulation will be retained.
As mentioned hereinabove, decompression expansion and rupture tends to occur when the insulation trys to expand due to the presence of compressed gasses trapped within it. Typically, the expanding gas tends to force the insulation outwardly in a radial direction causing conductor insulation on the outside conductors to press against the leading end of the cable and also against the exterior metal jacket or armor. Generally, however, the horizontal force component of these generated forces are balanced in the cable and no net displacement of insulation in the horizontal direction occurs. However, the components of forces in the vertical direction, that are radially-directed and hence, generally perpendicular to the longitudinal axis of the cable, tend to force apart the cantilevered parallel legs of the channel members. In extreme cases, high radial force components generated on decompression could drive apart the parallel legs of a channel member sufficiently to permit the cable insulation to edge flow around the slightly opened legs of the channel member and thus, rupture. The possibility of rupture is greatest for the central conductor because the jacket oftentimes merely spans the parallel legs of the channel members protecting this conductor and hence, does not apply direct restraint for the parallel legs of these central channel members.
However, the channel members located exteriorily and adjacent the outside edges of the cable structure receive radially inwardly-directed restraining forces from the cable jacket or armor because the corrugations thereof loop around the parallel legs of each of these channel members and provide loop strength to those members which resists the outward displacement of their parallel legs. Hence, these channel members need not be made as thick as the center channel members in order to withstand the same decompression forces.